This invention relates to electrical resistors having a glass-containing resistor film and more particularly to such resistors wherein the film lies in overlapping contact with metal film terminations of sub-micron thickness. The term sub-micron as used herein means thicknesses less than one micron and such sub-micron thick films are typically made by the well known process of metallo-organic deposition as is described and defined in a paper presented at the International Microelectronics Symposium, October 22-24, 1973, in San Francisco, (pp 4A 7-1 to 4A 7-8) by C. Y. Kuo entitled Electrical Applications of Thin-Films Produced by Metallo Organic Deposition.
Film resistors are commonly formed on an alumina substrate and typically serve as a part of a hybrid integrated circuit all of which is formed and interconnected on a single substrate.
Resistor films containing a noble metal conductive component are well known for their relatively inert and stable properties. They are often formed by a process that includes applying to a substrate a coating of a noble metal resinate paste and heating to decompose the resinate and form a noble metal/noble metal oxide resistor film. Such resinate derived resistor films are normally deposited in overlapping relationship with two or more conductive film terminations, which terminations have also been formed from noble metal resinates. The resistor, including terminations is usually though not always formed on a glazed ceramic substrate. Such glaze is called underglaze and serves to provide a smooth high melting temperature glass surface that does not substantially soften during the firing of the overlying components and which helps to assure predictable and high quality resistors that are formed thereon.
Resistor films may alternatively consist of a matrix of metal particles and glass. Such glass containing resistor films are generally orders of magnitude thicker than exclusively resinate derived films. Glass containing film resistor systems are generally capable of providing higher sheet resistivities, and tend to exhibit tight tolerances of temperature coefficient of resistance (TCR). Such glass containing film resistor systems are often preferred for these reasons.
It is well known that glass and metal particle containing conductive films must have a thickness greater than about 1.5 microns, since any attempt to reduce the film thickness by any means results in discontinuous non-conducting films. Such films are generally from 10 to 30 microns thick.
It is conventional to employ glass containing film terminations that provide strong termination substrate bonds for glass containing film resistors, even though it is well known that sub-micron thick film terminations containing only metals and/or metal oxides can be substantially more economical due to their relative thinness and high bulk conductivity. Much less metal is required in the equivalent termination. Such terminations, however, generally have a relatively poor bond to the underlying substrate and also contain high residual stresses. When an overlapping glass containing resistor film, typically containing a low temperature glass component, is subsequently formed, the low temperature glass components of the adjacent resistor system reacts at firing with the underglaze, lowering its melting point in the vicinity of the resistor-termination interface. It has been observed that the molten resistor glass either penetrates or moves underneath the thin-film termination of the region of overlap. The fluxing action of the resistor glass during the firing of the resistor film releases the termination from a glazed or unglazed ceramic substrate and allows the termination to shrink. This fluxing action is also effective in debonding the terminations of such a resistor when it is formed on a bare ceramic substrate having no intermediate underglaze, although not to so serious an extent. This leaves a gap, or at least a high resistance contact between the termination and the resistor film. This degredation of the termination is frequently accompanied by large cracks in the termination which in many cases cause it to become electrically open. The coating and firing of an overglaze may have a similar effect, it having been noted that the thin film termination floats in or even on top of the overglaze.
It is therefore an object of this invention to provide a sub-micron film termination in a resistor having a glass containing resistive element.
It is a further object of this invention to provide a low cost termination in a resistor having a glass containing resistor element.
It is a further object of this invention to provide a reliable sub-micron resistor termination that is covered with low temperature overglaze.